Concrete is a material which exhibits a low tensile strength and low fracture toughness. The ease with which cracks can nucleate and propagate in concrete under tension makes is imperative that concrete not be loaded in tension to the extent possible, and if unavoidable, some form of traditional reinforcement such as rebar be provided to take the tensile stresses. The latter is generally known as reinforced concrete and has been used for decades.
An alternate method of reinforcement is by incorporating short, randomly distributed fibers in concrete such that reinforcement throughout the volume is provided, and entirely new composite material is obtained. Fiber reinforced concrete is found to have significantly improved energy absorption capability (often called toughness), impact resistance and fatigue endurance, and its greater resistance to cracking also imparts it better durability and aesthetics.
U.S. Pat. No. 4,565,840 provides fiber reinforced concrete comprising from one to six percent by volume of a mixture of short steel fibers.
The addition of fibrillated plastic filaments to cement mortar is disclosed in U.S. Pat. No. 4,414,030. Such fibrillated filaments comprise ribbons having a length of up to 50 millimeters long and are said to be split apart during mixing with the mortar components to provide a random distribution of separate reinforcing plastic filaments throughout the mortar.
The use of discrete fibers in the reinforcement of concrete is set forth in U.S. Pat. No. 3,645,961. The patent discloses the use of nylon, polyvinyl chloride and simple polyolefins in lengths ranging between one-quarter to three inches (0.6 to 7.5 cm) to form a blast resistant concrete.
The use of fibrous materials made from nylon, polypropylene, poly-vinylidene chloride and polyethylene is set forth in U.S. Pat. No. 3,645,961. Less than 3 percent of these fibers in lengths from 1/4 to 3 inches (0.6 to 7.5 cm) can be mixed into concrete to make blast-resistant structures.
The use of fibrillated polypropylene fibers from 0.05 to 2 percent by weight of the total wet mixture of water-hardenable inorganic materials is presented in U.S. Pat. No. 3,591,395.
U.S. Pat. Nos. 5,456,752 and 5,628,822, owned by the Assignee of record, teach the use of graded synthetic fibers for the reinforcement of concrete. Gradation provides a plurality of different fiber types, i.e., lengths, deniers, widths, thicknesses, aspect ratios, cross-sections and fibrillations, in a controlled mixture adapted to accommodate the mortar factions in proportioned concrete.
According to the present invention, it is now understood that for a proper bond with the cementitious matrix around it, the fibers destined for concrete reinforcement must be deformed in geometry. However, most deformations put on commercial fibers are "ad-hoc" and little knowledge of what exactly constitutes an optimal deformation exists. The first attempt to rationally deform fibers was described is U.S. Pat. No. 5,443,918, which discloses metal fibers e.g., steel, having an elongated, substantially straight central portion and sinusoidally shaped end portions for addition to and reinforcement of cement-based material.
U.S. Pat. No. 4,585,487 discloses filiform or thread-like elements (fibers) of steel wire having uniform corrugations along their entire length for the reinforcement of concrete.
Bond-slip characteristics of fibers determined using a pull-out test (Banthia et al, "Concrete Reinforced with Deformed Steel Fibers, Part 1: Bond-slip Mechanisms" ACU Materials Journal, V. 91, No. 5, September-October 1994) are a wellaccepted way of assessing the performance of fibers in the composite.
The use of polymeric fibers (especially polypropylene) has to date remained limited to control of plastic shrinkage cracking in freshly placed concrete resulting from loss of mix and bleed water through evaporation. Given their non-structural purpose, the volume fractions of fibers used in these applications have also remained low (approximately 0.1%). The load carrying capacity of plain concrete without fiber reinforcement or that of concrete carrying a minimal amount of polypropylene fiber reinforcement (approximately 0.1% by volume) beyond matrix cracking is essentially zero. Lately though there have been some attempts to introduce greater volume fractions (about 1%) of larger diameter polypropylene fiber into concrete. These fibers have brought polymeric materials into the category of "structural" fibers where the purpose is not so much to control plastic shrinkage cracking, but also to improve the toughness, energy absorption capability and the load carrying capacity of concrete beyond first matrix cracking.
These new generation of "structural" polypropylene fibers are, however, straight and undeformed. As a result, they develop a poor bond with the surrounding matrix and are not very efficient. What is not known to-date, however, is what constitutes an optimal deformation for a low modulus material as polypropylene. Such an optimal deformation is the subject of the present invention.